1. Field of the Invention
The present invention relates to a clutch mechanism provided in a power-driven tool, such as a power-driven screwdriver and a power-driven hammer drill. More particularly, the present invention relates to a clutch mechanism for use in a power-driven tool that can adjust torque transmitted to the attached bit.
2. Description of the Prior Art
FIG. 6 shows a power-driven screwdriver 40 as disclosed in Japan Published Examined Patent Application No. S57-1393. In this screwdriver, the torque or a motor 41 is transmitted from a motor shaft 42 to a spindle 47 while being decelerated by a deceleration mechanism which includes planetary gears 45 engaged with internal gears 43 and 44 and a carrier 46 supporting the planetary gears 45. The torque eventually rotates a tool bit held by a chuck 48. The internal gear 44 is rotatably mounted in the deceleration mechanism and has on its front end a clutch face 44a. Balls 50 inserted in through-holes are formed in a gear case 49 which supports the spindle 47. The balls 50 are pressed against the clutch face 44a by a leaf spring 51. Provided in front of (toward the chuck 48) the leaf spring 51 is a clutch handle 52 with pressure protrusions 53 formed thereon. The clutch handle 52 is axially movable between a front position and a rear position where the pressure protrusions 53 are in abutment with the leaf spring 51.
When the clutch handle 52 is moved to the rear position, the pressure protrusions 53 presses the balls 50 against the clutch face 44a via the leaf springs 51. Even if a large load is applied to the spindle 47, the balls 50 cannot ride over the clutch face 44a, thus keeping the clutch engaged. In this so-called drill mode, the internal gear 44 continues to be prevented from rotating, so that the rotation of the motor 51 continues to be transmitted to the spindle 41 even if an excessive load develops.
When the clutch handle 52 is moved forward, the pressure protrusions 53 are detached from the leaf spring 51, allowing it to deform in the axial direction. The spring force of the leaf spring 51 sets the torque at which the internal gear 44 starts to rotate (this torque is referred to as "rotation-start torque" hereinafter). If a large load exceeding the rotation-start torque of the internal gear 44 is applied to the spindle 47, the balls 50 ride over the clutch face 44a. This causes the internal gear 44 to rotate idly. In this so-called screwdriver mode, the clutch is disengaged to prevent the rotation of the motor 51 from being transmitted to the spindle 41 if an excessive load develops.
Japan Published Unexamined Utility Model Application No. H6-39364 discloses another power-driven tool with a torque adjustment device. The tool comprises a compression spring for biasing a plate backward against balls (corresponding to the balls 50 in the foregoing example). The tool also includes a spring holder for adjusting the compression of the spring. When the spring is fully compressed, the tool is placed in the drill mode, in which the balls do not ride over the clutch face. When the spring is not fully compressed, the tool is placed in the screwdriver mode with the rotation-start torque corresponding to the spring compression set for the internal gear. If a load exceeding this rotation-start torque is applied to the internal gear, the balls ride over the clutch face, thereby causing the internal gear to rotate idly.
In either of the aforementioned clutch mechanisms, however, if the tool is mistakenly used as a screwdriver when the drill mode is selected, the spindle is locked as the screw is tightened since the clutch cannot be disengaged, causing an abrupt impact to be transmitted from the tool bit to the tool mechanism, sometimes damaging the gear case, the planetary gears, and other parts of the tool. Moreover, jolts are transmitted to the operator's hand.